The radioactive hydrogen isotope essential to near-term fusion reactors, tritium must be bred on-site from lithium because no natural supply exists at scale.
Tritium (symbol T or 3H) is a radioactive isotope of hydrogen with one proton and two neutrons. It decays by beta emission to helium-3 with a half-life of 12.32 years. Tritium is one of the two fuels in the deuterium–tritium (D–T) fusion reaction, which has the largest cross-section at temperatures achievable in magnetic confinement devices.1
The D–T reaction releases 17.6 MeV per event — more energy at lower temperature than any other fusion fuel combination. This makes it the only reaction likely to achieve net energy gain in first-generation power plants. However, tritium does not occur naturally in useful quantities. Cosmic-ray interactions with the atmosphere produce only about 4 kg per year globally, and the world's current civilian inventory — mostly from CANDU heavy-water reactor operations — totals roughly 25–30 kg.2
In a fusion reactor, 14.1 MeV neutrons from the D–T reaction enter a lithium-containing blanket surrounding the plasma. Two breeding reactions are exploited:
A tritium breeding ratio (TBR) greater than 1.0 is required for fuel self-sufficiency. Neutron multipliers such as beryllium or lead are incorporated into blanket designs to boost the TBR.3
Tritium is a low-energy beta emitter (endpoint energy 18.6 keV) and is not an external radiation hazard — it cannot penetrate skin. The primary risk is internal exposure through inhalation or ingestion of tritiated water. Fusion facilities therefore employ multiple confinement barriers, atmospheric detritiation systems, and rigorous accountancy.4
Today, tritium is produced as a byproduct in CANDU reactors (Ontario Power Generation is the world's largest supplier) and in the U.S. Department of Energy's tritium-producing burnable absorber rod program at Watts Bar. The transition from external supply to self-sufficient breeding is one of the defining engineering challenges for the fusion industry.